The present invention relates to receiver systems having a particularly advantageous application for receiving pulse trains coming from satellites, e.g. for the purpose of determining the position of a land, sea, or air vehicle having the system on-board, i.e. systems known under the initials "G.P.S.".
One such receiver system for receiving pulse trains transmitted by satellites is described in EP-A-0 166 911, in particular.
In brief, that receiver system comprises a receiver for receiving pulse trains transmitted by the satellite, a demodulator whose first input is connected to the output of the receiver means, two generators: a code signal generator at the frequency of the pulse trains transmitted by the satellites and a Doppler shift carrier signal train generator. The outputs of those two generators are connected respectively to second and third inputs of the demodulator, the signals delivered by the demodulator serving, for example, to determine the position of a vehicle including such a receiver.
Other systems for receiving pulse trains transmitted by satellites, in particular, are described in numerous other documents, for example in the article entitled "A GPS fast acquisition receiver" published in the journal IEEE 1983 National Telesystems Conference, San Francisco, Calif., US, 14th-16th Nov. 1983, pages 214-218, and in the article entitled "Receivers for the NAVSTAR global positioning system" published in the journal Proceedings of the IEE-F Communications Radar and Signal Processing, Vol. 127, No. 2, April 1980, pages 163-167.
The use of such receiver devices is becoming more and more common, particularly because of the density of the network of satellites to be found in geostationary positions around the earth, since the signals which are transmitted by such satellites can be received on the earth and can be interpreted, e.g. by the so-called "triangulation" method, to determine the position of the point at which they have been received.
To implement the system briefly outlined above, it is necessary firstly for the satellites to send information repetitively in the form of digital pulse trains that are encoded using a coding systems relating to each satellite, and secondly for there to exist receiver systems suitable for receiving said information as transmitted by the satellites and for decoding it. Such receiver systems can be located, for example, on board vehicles whose position relative to the earth needs to be established. This applies, for example, to ships at sea or to aircraft or to other vehicles that move in the atmosphere.
That technique gives good results, however implementation thereof is relatively lengthy. The coded pulse trains transmitted by the satellite are relatively long in duration, e.g. several seconds, and in order to identify the satellite transmitting the pulses it receives, a "G.P.S." receiver must as a general rule process a plurality of successive pulse trains transmitted by said satellite so as to be capable firstly of identifying the satellite and secondly of extracting the data required for deducing the desired information therefrom.
In some cases, e.g. for determining the position of a ship at sea, the error in the position of the ship due to the time taken to implement the technique is of no consequence since the vehicle is moving at a relatively low speed. However that is not true of vehicles such as aircraft flying at high speed, or rockets, etc., where such errors can give rise to sever consequences such as accidents, collisions, etc.